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  Electrolytic apparatus for use in oxide electrowinning methodUSPTO Application #: 20060151326Title: Electrolytic apparatus for use in oxide electrowinning method Abstract: An electrolytic apparatus for an oxide electrolytic method having a constitution such that in the interior of an electrolytic vessel 10, a common cathode 12 and two types of anodes different in shape and arrangement (here, a first anode 14 arranged beneath the cathode, and a second anode 16 arranged in parallel to the cathode) are provided; a first electrolysis controller 18 is connected between the cathode and the first anode, and a second electrolysis controller 20 is connected between the cathode and the second anode. The electrolytic processing of the substance 22 to be processed in the electrolytic vessel is carried out in such a way that a pair of the cathode and one of the anodes is used for main electrolysis and a pair of the cathode and the other anode is used for auxiliary electrolysis. By this apparatus, prevention of the ununiform distribution of the electrodeposit, improvement of the processing speed and improvement of the durability of the crucible are achieved, whereby the recycling of spent nuclear fuels based on the nonaqueous reprocessing method is made feasible in a commercial scale. (end of abstract) Agent: Wenderoth, Lind & Ponack, L.L.P. - Washington, DC, US Inventors: Kenji Koizumi, Nobuo Okamura, Tadahiro Washiya, Shinichi Aose USPTO Applicaton #: 20060151326 - Class: 205043000 (USPTO) Related Patent Categories: Electrolysis: Processes, Compositions Used Therein, And Methods Of Preparing The Compositions, Electrolytic Process Involving Actinide Series Elements Or Compound (at. No. 89+) (product, Process, Composition, And Method Of Preparing Composition) The Patent Description & Claims data below is from USPTO Patent Application 20060151326. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention relates to an electrolytic apparatus for use in an oxide electrowinning method in which apparatus plural types of anodes and at least one common cathode are provided, and the control of the electrodeposit is made to be efficiently carried out by using a pair of one of the anodes with the cathode for main electrolysis and a pair of the one or more remaining anodes with the cathode for auxiliary electrolysis. This technique is useful for the electrolytic system in the oxide electrowinning method, among the nonaqueous reprocessing methods with a molten salt electrolytic technique for spent nuclear fuels. BACKGROUND ART [0002] Researches have been developed on a system to achieve the improvement of the economical efficiency of a whole recycling system in which uranium and plutonium are recovered by utilizing a molten salt electrolytic technique as a reprocessing technique for recycling of spent nuclear fuels used in nuclear reactors. The molten salt electrolytic technique is expected to be high in economical efficiency. (See, for example, Japanese Patent Laid-Open Specification No. 2001-141879.) The relevant electrolytic techniques include an oxide electrowinning method and a metal electrorefining method. When the chemical forms of uranium and plutonium in the electrodeposit are oxides, the oxide electrowinning method is employed. [0003] The oxide electrowinning method is to recover oxides of uranium and plutonium through a simultaneous electrolytic step, a dissolution step by chlorination and a MOX recovery step. In this method, the spent nuclear fuel is first placed in the bottom portion of a crucible doubling as an anode, and then electrolysis is carried out between the anode and a cathode installed in an upper portion of the crucible. By this operation, uranium oxide contained in a large amount in the spent nuclear fuel is dissolved into the molten salt due to anodic oxidation, and simultaneously recovered by depositing uranium oxide on the surface of the cathode due to cathodic reduction (a simultaneous electrolytic step). Thereafter, the electrolytic operation is stopped, and uranium oxide, plutonium oxide and other elements remaining in the spent nuclear fuel are dissolved into the molten salt by blowing chlorine gas into the molten salt to convert them to chlorides thereof (a dissolution step by chlorination). After the whole spent nuclear fuel has been dissolved into the molten salt, electrolysis is carried out between the anode doubling as the crucible and the cathode installed in the upper portion of the crucible, and the oxides of uranium and plutonium are recovered by depositing the oxides in a mixed state on the surface of the cathode (a MOX recovery step). [0004] The reactions involved in the respective steps are shown below: [0005] a simultaneous electrolytic step: UO.sub.2.fwdarw.UO.sub.2.sup.2+ (anodic reaction) UO.sub.2.sup.2+.fwdarw.UO.sub.2 (cathodic reaction) [0006] a dissolution step by chlorination: UO.sub.2+Cl.sub.2.fwdarw.UO.sub.2Cl.sub.2 PuO.sub.2+C+2Cl.sub.2.fwdarw.PuCl.sub.4+CO.sub.2 [0007] a MOX recovery step: UO.sub.2Cl.sub.2.fwdarw.UO.sub.2+Cl.sub.2 (cathodic reaction) PuCl.sub.4+O.sub.2.fwdarw.PuO.sub.2+2Cl.sub.2 (cathodic reaction) [0008] As described above, in the conventional technique, there is adopted a constitution such that the crucible containing the substance to be treated doubles as the anode, the cathode is installed in the molten salt, and electrolysis is carried out between the anode (the crucible) and the cathode. Alternatively, there is another constitution such that the anode and cathode are installed in the crucible and electrolysis is carried out therebetween. [0009] However, such a conventional technique as described above has suffered from the following problems to be solved. When the crucible doubles as the anode, in the steps other than the simultaneous electrolytic step, the distance between the anode and the cathode is maintained to be uniform and hence the current density is uniform, so that the ununiform distribution of the electrodeposit can hardly take place; on the contrary, in the simultaneous electrolytic step, the spent nuclear fuel placed in the bottom of the crucible functions as the anode, so that the distance between the electrodes is not maintained constant. Consequently, the current density distribution on the surface of the cathode becomes ununiform, resulting in the ununiform distribution of the electrodeposit. Further, the distance between the lower end of the cathode and the surface of the spent nuclear fuel becomes shorter, the current density around the lower end of the cathode is thereby increased and accordingly the electrodeposit is concentrated around the lower end of the cathode, so that when stirring is not sufficiently conducted, the ions in the bulk region come to be insufficient and the processing speed is degraded. [0010] Additionally, because the environment involved is highly corrosive owing to the use of chlorine gas, there is used a material prepared by coating (with vapor deposition) graphite blank with pyrographite excellent in corrosion resistance as the material for the crucible doubling as the anode. However, because of the operation condition such as high temperature molten salt and chlorine gas conditions, the operation life time of the crucible is in the order of 1,000 hours. Consequently, the crucible need be replaced at frequent intervals, leading to the decreasing of the processing speed. [0011] Furthermore, it is conceivable that the electrolytic apparatus is made larger in size as a measure for improving the processing speed. However, the crucible made of pyrographite can hardly be made larger in size because of the viewpoint of product fabrication. [0012] Even when the constitution is such that the anode and cathode are installed in the crucible, the distance between the electrodes are not uniform, and hence the current density distribution on the cathode surface comes to be ununiform, and the ununiform distribution of the electrodeposit takes place. [0013] The bonding force between uranium oxide and plutonium oxide deposited as the forms of oxides and the surface of the electrodes are lower than the bonding force for the metallic state as in plating and the like. Consequently, in the conventional technique, in any case where the electrodeposit is concentrated in a particular portion, the possibility that the electrodeposit falls down from the surface of the cathodes during the electrolytic operation becomes high owing to the stirring effect of the various process gases blown into the molten salt. [0014] Additionally, in view of the prevention of the criticality, it can hardly be an appropriate countermeasure to simply make the electrolytic apparatus larger in size. DISCLOSURE OF THE INVENTION [0015] An object of the present invention is to provide an electrolytic apparatus for use in an oxide electrowinning method which apparatus can prevent the ununiform distribution of the electrodeposit. [0016] Another object of the present invention is to provide an electrolytic apparatus which can achieve the improvement of the processing speed and the improvement of the durability of the crucible, and can carry out the recycling of spent nuclear fuels in a commercial scale on the basis of the nonaqueous reprocessing method. [0017] According to the present invention, there is provided an electrolytic apparatus for use in an oxide electrowinning method, the apparatus comprising a plurality of anodes different from each other in shape and arrangement and at least one common cathode installed in an electrolytic vessel, wherein a pair of one of the anodes and the cathode is used for main electrolysis and a pair of the one or more remaining anodes and the cathode is used for auxiliary electrolysis. [0018] Additionally, there is provided an electrolytic apparatus for use in an oxide electrowinning method, the apparatus comprising an annular electrolytic vessel made of a metallic material and designed in consideration of criticality control with geometrical control, a high frequency induction coil for heating a substance to be processed in said electrolytic vessel, an annular anode installed at the bottom of an annular space formed in the annular electrolytic vessel, and rod-shaped anodes and rod-shaped cathodes installed along the axial direction in the annular space, wherein a parallel pair of the rod-shaped anodes and the rod-shaped cathodes arranged in parallel or a vertical pair of the annular anode and the rod-shaped cathodes arranged vertically is used for main electrolysis and the other of the pairs is used for auxiliary electrolysis. [0019] A typical example of the parallel pair of electrodes is a constitution of alternately arranged electrodes in which the anodes and cathodes are alternately arranged. As for these arrangements, preferable is a constitution wherein the rod-shaped cathodes are supported rotatably and a rotation driving mechanism is additionally installed, and the cathodes are continuously rotated during electrolytic operation. [0020] Additionally, the present invention is a spent nuclear fuel reprocessing method with an oxide electrowinning method by using such an electrolytic apparatus as described above, wherein the substance to be processed in the annular electrolytic vessel is a molten salt dissolving the spent nuclear fuel, and wherein in a simultaneous electrolytic step in which uranium oxide contained in the spent nuclear fuel is dissolved into the molten salt by anodic oxidation reaction and simultaneously recovered as uranium oxide electrodeposition on the surface of the cathode by cathodic reduction reaction, the vertical pair of the electrodes is used for main electrolysis in which uranium oxide is dissolved and deposited by electrochemical reaction, and the parallel pair of the electrodes is used for auxiliary electrolysis whose role is to suppress the ununiform uranium oxide electrodeposition; and in a MOX recovery step in which the oxides of uranium and plutonium are deposited and recovered in a mixed state, the parallel pair of the electrodes is used for main electrolysis in which MOX is deposited, and the vertical pair of the electrodes is used for auxiliary electrolysis whose role is to dissolve the electrodeposit fallen down from the cathodes. 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